An inorganic electroluminescent element has been used as a flat type light source. An alternative high voltage is required to drive such the inorganic electroluminescent element.
A recently developed organic EL element is an element has a structure in which an organic compound thin layer containing an organic fluorescent compound is arranged between a cathode and an anode. The EL element emits light, fluorescence or phosphorescence, when the excitons formed by recombination of electrons and holes injected into the thin layer are deactivated. To utilize the emitted light, at least one of the electrodes each arranged at both sides of the organic compound thin layer is usually a transparent electrode such as ITO. The transparent electrode is supported by a transparent substrate such as a glass plate.
The organic EL element is noted from the viewpoint of the portability and space saving since the element can emit light at a low voltage within the range of from several to several decade volts and has a wide viewing angle and a high visuality since the element emits light itself and is complete solid thin layer shaped element.
Various full color displaying apparatus and projecting apparatus each using the organic EL element have been proposed. Among them, Japanese Patent Publication Open to Public Inspection, hereinafter referred to as JP O.P.I., No. 3-152897 discloses a displaying apparatus to display a full color image by the use of an organic EL element composed of an organic compound thin layer containing a blue or blue-green light-emitting layer arranged between the electrodes on which two color conversion members and a blue color filter for raising the purity of blue light are provided. One of the color conversion members is a member emitting green fluorescent light and the other is a member emitting red fluorescent light by absorbing the blue of bluish green light emitted from the organic EL member. The displaying apparatus of such the system is superior at the point of easy production to an usual displaying apparatus in which three kinds of organic EL members each emitting blue, green and red light are separately coated. However, it is a problem that red light having a sufficient intensity is difficultly obtained through the color conversion member since the light emitted from the organic EL member is blue or blue-green.
Moreover, the light is largely lost when the light is conducted from the light emission member to the light conversion member since the light emitted from the light-emitting layer has no directivity and scatters in all directions. The light only emitted to the side of the color conversion member, the front direction, from the organic EL member is utilized, and the output coefficient of light to the front direction, the light emission coefficient, is defined by the refractive index n of the light-emitting layer since the light emission coefficient can be approximated by ½n2 according to the classical optics. The light emitting coefficient of the organic EL member is about 20% when the refractive index of the light-emitting layer is 1.7. The light other than the above is transported in the area direction of the light-emitting layer, the scatter in the side direction, or disappeared in the metal electrode facing to the transparent electrode through the light-emitting layer. Accordingly, the intensity of the light is made insufficient and the displayed image becomes too dark.
The foregoing problem of the low light emitting coefficient is a common problem should be solved in the apparatus using the organic EL element not only the apparatus disclosed in the above JP O.P.I. No. 3-152897.
The color purity of light emitted from the organic fluorescent compound contained in the light-emitting layer of the organic EL element is not always high since the half band width of the spectrum of the emitted is about 100 nm.
In such the situation, an organic EL element having a micro optical resonator structure is proposed as a method for solving the problems of the light emission coefficient and the color purity in JP O.P.I. Nos. 6-283271, 7-282981, 9-180883 and 2000-98931. In such the elements, a dielectric mirror is provided between the transparent electrode and the transparent substrate so that the light is intensified by the interference of light between the metal cathode as the light reflection mirror and the dielectric mirror when the light is emitted from the light-emitting layer placed between these mirrors. In the organic El element having the micro optical resonator structure, the thickness of the layer provided between the foregoing mirrors can be optionally designed so that the interference condition is satisfied at a desired wavelength of light or the emission light wavelength. Therefore, the intensity of the light at the designed wavelength is intensified and the content of the other wavelength is inhibited. Accordingly, the half band width of the emitted light spectrum is decreased and the color purity of the light can be raised. Furthermore, the pattern of light emission is become to strongly depend on the wavelength of the emitted light so that the light at the emission wavelength is considerably directed to front of the dielectric mirror and the loss in the side direction is decreased.
The light emission mechanism of the organic EL element having the micro optical resonator structure is described in “Control of Light Emission Characteristics of Organic EL Element”, S. Tokitou, NHK Broadcasting Technological Laboratory, Reference material of lecture meeting, and “Device Structure and Light Emission Mechanism of Organic EL Element II”, A. Fujii and K. Yoshino, Applied Physics, Vol. 69, No. 11, 2000.
However, a problem that the wide visible angle cannot be obtained because of the directivity of the emitted light is occurred when the organic EL element having the micro optical resonator structure is applied for the displaying apparatus. Therefore, the application for the projection apparatus has been proposed only. Furthermore, there is a problem on the color reproduction that the color of the emitted light is contained because of that the optical length of the system is varied depending on the viewing angle.
On the other hand, it is pointed out as the problem on the organic EL element that the low durability of the element caused by the low stability of the element against the moisture of air. In the present circumstances, a transparent glass with a low moisture permeability is used as the transparent substrate of the organic EL element and the element is formed on the substrate by a dry layer forming method such as a deposition method, moreover, the element is shielded by a cap of metal or glass. However, the cost of the element is raised because of the low productivity, and the element is difficultly applied to a portable apparatus such as a PDA since the element with no flexibility is easily broken and the weight thereof is large.
It is considered, therefore, to use a transparent resin film as the transparent substrate. However, a film of poly(ethylene terephthalate), poly(ethylene naphthalate) and cellulose triacetate cannot be used as the transparent substrate different from the glass plate since such the resin film has a high moisture permeability.